treewide: add support for "lto" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to enable
it treewide, where chosen packages can still opt-out via "no-lto".

Some packages used LTO, but not the linker plugin. This unifies 'em
all to attempt to produce better code.
Quoting man gcc(1):
"This improves the quality of optimization by exposing more code to the
link-time optimizer."

Also use -flto=auto instead of -flto=jobserver, as it's not guaranteed
that every buildsystem uses +$(MAKE) correctly.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/system/procd/Makefile

@@ -22,6 +22,7 @@ PKG_LICENSE_FILES:=
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 
+PKG_BUILD_FLAGS:=lto
 PKG_ASLR_PIE_REGULAR:=1
 PKG_CONFIG_DEPENDS:= \
 	CONFIG_TARGET_INIT_PATH CONFIG_KERNEL_SECCOMP CONFIG_PROCD_SHOW_BOOT \
@@ -36,9 +37,6 @@ endif
 
 CMAKE_OPTIONS += -DEARLY_PATH="$(TARGET_INIT_PATH)"
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto
-
 define Package/procd/Default
   SECTION:=base
   CATEGORY:=Base system